Substance P is a neuropeptide that can affect host responses in peripheral tissues and in the central nervous system. As such, substance P and its receptor, the neurokinin-1 receptor, represent one of the most important links between the nervous and immune systems. Neuro-immunological interactions will be the focus of this proposal, with the overall goal of clearly defining the mechanisms used by neurokinin-1 receptors to augment the host response against pathogens. These investigations will make use of in vivo and in vitro models of infection, and focus on macrophages, dendritic cells, and microglia, and how the neuropeptide substance P can augment host-derived or pathogen-derived signals. The use of neurokinin-1 receptor-deficient mice will provide a valuable in vivo model system to define differences in the host responses when compared to congenic, wild-type mouse strains. Bacterial and viral pathogens will be used to define in vivo mechanisms mediated by substance P binding to neurokinin-1 receptors. Such mechanisms will focus on the initiation of host responses, as well as the development of antigen-specific responses. Studies will also focus on the effects of substance P on antigen processing/presentation and co-stimulation mediated by neurokinin-1 receptors on macrophages and dendritic cells as a mechanism for modulating the host response. In vitro cultures of primary brain microglia will be investigated for their responsiveness to substance P. In addition, microglia cultures will be used to question whether neurokinin-1 receptor-mediated mechanisms can be defined which might help to explain neuropeptide-mediated inflammatory responses within the central nervous system. The methods which will be used in these studies include ribonuclease protection assays, enzyme-linked immunosorbent assays of molecules present in T-PER homogenates, flow cytometric analyses of immunofluorescently stained cells, and confocal analyses of tissue sections which have been tri-color stained to identify the cellular source of expressed molecules. Genetically deficient and transgenic mice will also be used as models for defining substance P mediated responses. Taken together, these studies will demonstrate that substance P is an integral part of the host immune response against pathogens.